Increased activity in offshore drilling and completion has caused an increase in working pressures such that new offshore wells, including some that are being drilled off the coast of Canada and in the North Sea in depths over 300 feet, have working pressures of as high as 15,000 psi. Drilling operations in these wells generally include a floating vessel having a heave compensator for a riser and drill pipe extending to the blowout preventer and wellhead located at the mud line. The blowout preventer stack is generally mounted on 20 inch pipe with the riser extending to the surface. A quick disconnect is often located on top of the blowout preventer stack. An articulation joint is used to allow for vessel movement. One problem related to the subsea wells having such high working pressures approaching 15,000 psi is to provide an energizing means for setting a sealing means between the casing hangers and the wellhead which will withstand and contain such working pressure, without subjecting the drill string and related apparatus at the surface and at the wellhead to unduly high or excessive torque. It is an object of the present invention to provide for such an energizing means that is simple, easy to manufacture, easy to install and retrieve, and reliable.
Energizing means, in general, for energizing and setting a sealing means between a casing hanger and a wellhead in an underwater oil or gas well are well known. See, for example, the energizing means disclosed in U.S. Pat. Nos. 3,054,449, 3,357,486, 3,543,847, 3,693,714 and 3,933,202.
One common method of actuating such a sealing means is by applying weight force on the sealing member, for example, via drill collars, to expand it in the annulus. Weight energizing means is the least desirable because the handling of drill collars providing the weight required for high sealing pressure is difficult and time consuming on the rig floor. Weight energizing means are disclosed in U.S. Pat. Nos. 3,054,449; 3,543,847, combined with hydraulic pressure actuation means, hereinafter described; and U.S. Pat. No. 3,933,202.
Another method for actuating such a sealing means is by applying hydraulic pressure from the surface to the underwater wellhead to expand and set the sealing assembly in the annulus between the casing hanger and the wellhead housing. If hydraulic pressure is applied through the drill pipe, one drawback is that there is a need for wireline equipment to run and recover darts from the hydraulically activated seal energization system. If darts are not used, the handling of "wet strings" of drill pipe is very messy and unpopular with drilling crews. If the seal energization means uses the single trip casing hanger technique, the cementing fluid can cause problems in the hydraulic system used to energize the seal. Furthermore, maintenance is also a problem when hydraulic activated systems are used. Hydraulic pressure activated systems are disclosed in U.S. Pat. Nos. 3,357,486; 3,543,847, combined with aforementioned weight activation means; and U.S. Pat. No. 3,693,714.
The most desirable method to energize a seal is by applying torque on the drill string extending to the underwater wellhead which in turn rotates and advances on threads a packing nut which axially compresses and radially expands the seal assembly in the annulus between the casing hanger and the wellhead. However, it is desirable to minimize the amount of torque applied to the drill string to reduce the wear and tear on the torque generating apparatus at the surface and the drill string itself and therefore, to reduce the chances of failure. Moreover, there are limitations on the amount of torque which can be transmitted from the surface due to friction losses and the like.
The tool of the present invention enables one to set a seal to withstand the high working pressures discussed above which are expected in some new underwater oil or gas wells by applying to the drill string a torque which is substantially lower than the torque ultimately applied by the tool on the sealing assembly. Thus, the sealing assembly can be torque set to withstand the higher working pressures while limiting the torque applied to the drill string to an amount that does not unduly stress the drill string and related apparatus used to run the casing hanger and sealing assemblies into the well and to set the seal.
Other objects and advantages of the invention will appear from the following description.